Add support for generic version label parameter in plugins xml

[Xpirix]

Fix for #448

• Added support for generic label parameters (latest, ltr and stable) in the plugin xml.
• When using one of these parameters, the view will get the numbered version from https://version.qgis.org/version.txt or version-ltr.txt (depending on the parameter).
• Redirect to the same view but using the numbered version as we already have them cached
• Note: The full version with patch is not supported so we only use the major and minor version here.

generic-label-support.mov